Many foodstuffs are heated, cooled, frozen or otherwise treated thermally by holding them in, or passing them through, the parallel gaps between adjacent horizontal plates of an horizontal plate heat exchanger, for example an horizontal plate freezer; or by holding the foodstuff in the vertical gap between adjacent vertical heat exchanger plates, for example of a vertical plate freezer. In the case of a horizontal plate freezer, the foodstuff can be loaded and unloaded manually or automatically and this form of freezer is of primary use in the treatment of discrete individual units of the foodstuff, for example containers of prepared meals and the like. In the vertical plate freezer, the foodstuff is usually loaded into the vertical chamber between adjacent plates manually and as a fluent mass of the foodstuff, for example as a slurry, as with a fruit juice or puree, or as a mass of small units of the foodstuff which are not kept discrete, as with a mass of loose fish or meat which is to form a frozen block. For convenience, the invention will be described hereinafter in terms of vertical heat exchange plates, notably a vertical plate freezer, although it can equally be applied to a horizontal plate freezer or other heat exchanger.
Heat exchange fluid flows through bores or conduits within the structure of the heat exchange plate to supply heat to or remove heat from the foodstuff. Such plates are usually large, typically ranging from 0.5 to 2.5 metres by from 1 to 4 metres and are made by extruding the plate from one or more blocks of solid metal. Initially, such plates were made from mild steel, but this was replaced by the use of aluminiumas aluminium extrusion techniques became accepted in the metal fabrication industry.
However, despite the advantageous thermal properties of aluminium, its use gives rise to problems. Due to the ease with which aluminiumforms a food-soluble oxide/hydroxide on its surface when left in contact with food for prolonged periods, care has to be taken during the use and cleaning of aluminium freezer plates between uses to minimise the risk of such corrosion and contamination occurring. In some countries, for example the USA, the use of bare aluminiumin contact with foodstuffs under conditions where contamination of the foodstuff could occur presents particular health problems. In some applications, for example in the canning of food or beverages, it is possible to apply a protective plastic or other coating to the exposed aluminium surface. However, where solids are being handled, as with blocks of foodstuff or meal portions in metallic containers, abrasion and erosion of surface coatings will occur, notably on load-bearing surfaces as present in a plate freezer. Such coatings are not therefore practical in such applications.
Stainless steel is corrosion resistant and finds widespread use in the food processing industry. Stainless steel cannot be cast or extruded and must therefore be machined or pressed where complex shapes are required. However, stainless steel is difficult to machine and its large scale use has been restricted to applications where the metal can be formed into the desired shape by pressing, for example in heat exchanger plates which are pressed out from sheet metal. It has been considered impractical and uneconomic to manufacture a freezer plate wholly from solid stainless steel.
It has been proposed to fabricate a freezer plate from a serpentine tube, which is to carry the heat exchange fluid, sandwiched between galvanised steel or stainless steel sheets. However, since the tube contacts the adjacent sheets along only a narrow line of contact, such plates give poor and uneven heat transfer from the tube to the sheet. This results in high energy input requirements to operate the plate freezer effectively, and in localised areas of excessive and/or insufficient freezing of the foodstuff in contact with the sheets. In general, such plate freezers have been superseded by the use of extruded aluminium plates which give more uniform freezing of the foodstuff and, due to the high thermal conductivity of aluminium as compared to steel or stainless steel, give greater heat efficiencies.
In an attempt to overcome the poor and uneven heat performance of such sandwich type plates, it has been proposed to fill the internal space in the plate between the tube and the sheets with a eutectic material. Whilst this aids heat transfer between the tube and the sheets and provides a more even thermal transfer over the surface of the sheets, such a system is not suitable for use in vertical plate freezers. In such freezers, the foodstuff is usually frozen as a solid block between the plates. Such a block readily adheres to the faces of the plates and cannot be removed. It is therefore necessary to pass a heating medium through the plate partially to melt the block of frozen material immediately adjacent to the plate surface. With a plate filled with a eutectic fluid, the thermal properties of the fluid prevent rapid heating and cooling of the sheets forming the external faces of the plate. Therefore, any re-heating of the cold plate will occur slowly and extensive partial thawing of the foodstuff block between the plates will occur, which is unacceptable.
Due to the problems of fabrication and thermal efficiency with stainless steel, and despite the problem of potential food contamination, aluminium continues to be used as the material from which plate freezers plates are manufactured. In those cases where unacceptable contamination problems could arise, it has been accepted in the food processing industry that alternative methods for freezing foodstuffs have to be used. Thus, the foodstuff is usually packed in individual closed containers, for example metal cans, and the contents of the containers are frozen by holding the containers in a cold store for several days, which is clearly inefficient and time consuming.
We have now devised a form of freezer plate construction which reduces the above problems.